Background biology

A brief guide to DNA, genes, genomes and other biology basics

  1. What is a genome?
  2. What is DNA sequencing?
  3. What's in a DNA sequence?
  4. What’s the relationship between genetics and genomics?

1. What is a genome?

A genome is a complete set of instructions to make and operate an organism. Your genome contains the information (encoded in your DNA) to build your cells, your cells’ components and their components’ components. It also carries the information necessary for the cells to function.

2. What is DNA sequencing?

DNA sequencing is a laboratory technique used to determine the sequence of DNA units — represented by the letters A, C, G, and T — in a DNA molecule. Sequencing methods have changed over time; Illumina’s machines use complex chemistry and high-resolution optics to determine the sequence.

In a sequencing laboratory, machines break the DNA up into manageable segments and read the order of the DNA bases or letters. Computers are then used to compare the DNA sequence with other sequences to locate the differences or variants.

The cost of DNA sequencing has dropped dramatically in the last decade. The first human genome sequences took years and cost more than $1 billion to produce. Now, sequencing technologies can sequence a person’s genome in days.

3. What’s in a DNA sequence?

The DNA sequence itself is a series of letters — A, C, G, and T — that represent the order of DNA units, or bases, that lie along the length of a person’s DNA. Because each person inherits one copy of their chromosomes from their mother and another from their father, there are technically more than 6,000 million letters to read and interpret. However, people’s genomes are remarkably similar, so only around 3-4 million differences are usually found between any two people alive.

In general, researchers are most interested in:

  • Genes: lengths of DNA that carry the information to produce other molecules, often proteins
  • Regulatory mechanisms: sequences or chemical modifications of the DNA that control the activity of different parts of the genome, at different times, in different cells.
  • Sequence variants: differences between you and other people, and differences between different types of cells (eg. tumour cells and non-tumour cells). Important variants can occur within genes, or in the sections of DNA. Some variants are single letter differences; others can be long stretches of DNA that have been shifted about, inserted, or deleted.

Variants can provide different types of information about an individual:

  • about their family and ancestry; 
  • about their health now or in the future; and
  • about the types and dosage of drugs that may work best for them.

However, our knowledge of the genome is far from complete. Researchers are unsure of the role that most gene variants play in health and disease. Understanding more about these variants — and when and why they occur — will help us better understand the causes of diseases such as cancer, and help improve their diagnosis and treatment.

4. What’s the relationship between genetics and genomics?

Both genetics and genomics are concerned with genetic material. 

Genetics is concerned with heredity — the way characteristics are passed from one generation to the next -- and the function of specific segments of DNA — genes. Genomics considers many genes at once to investigate their combined influence in an organism.

In genetic testing, only a specific location or section of the genome is being analysed. Genomic testing provides more data and may be more cost effective, but that means there’s more to analyse and interpret as well.

For some research questions, it’s necessary to determine the order DNA bases across the genome. For other questions, only a targeted subset of these genes are analysed.